bass clef

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A symbol indicating that the pitch of the second highest line of the staff is F below middle C.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music A symbol showing that the second line from the top of the staff represents the F below middle C.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a clef that puts the F below middle C on the fourth line of a staff